This invention relates to programmable logic devices, and more particularly to driver circuitry usable in the interconnection resources of such devices.
A typical programmable logic device has a plurality of regions of programmable logic disposed on the device in a two-dimensional array of intersecting rows and columns of such regions. Each region may include a plurality of subregions of programmable logic. Each subregion may include (1) a four-input look-up table which is programmable to produce an output signal that is any logical combination of the four inputs applied to the look-up table, (2) a register (flip-flop) for registering the output signal of the look-up table, and (3) circuitry for allowing the final output of the subregion to be either the registered or unregistered output signal of the look-up table. Interconnection conductors are provided on the device for conveying signals to, from, and between the subregions in each region, as well as to, from, and between the regions. For example, horizontal interconnection conductors may be associated with each row of regions for conveying signals to, from, and between the regions in the associated row. Vertical interconnection conductors may be associated with each column of regions for conveying signals to, from, and between the rows. And local conductors may be associated with each region for conveying signals to, from, and between the subregions in that region. Programmable interconnections are provided for making connections between the various types of interconnection conductors so that signals can be routed throughout the device in a great many different ways. For example, the local conductors associated with each region may be programmably interconnectable to the horizontal and/or vertical conductors adjacent to that region. Similarly, intersecting horizontal and vertical conductors may be programmably interconnectable.
Various kinds of drivers may be provided for driving signals from the subregions out onto the adjacent interconnection conductors. For example, certain of the horizontal and vertical conductors adjacent to each region may be driven by the output signals of that region's subregions via a buffer and an NMOS pass gate. Each such buffer may be capable of driving one or more horizontal and/or vertical conductors. Each pass gate is controlled by an associated static programmable element. Alternative driver circuitry involves the use of tri-state drivers feeding tri-state lines. The enable signal for each tri-state buffer is generated elsewhere on the device or comes from an input pin. Thus each such enable signal must be explicitly routed to each tri-state driver that it controls. This can result in extra delay in the enable path and may require considerable routing resources.
In view of the foregoing, it is an object of this invention to provide improved driver circuitry for programmable logic devices.
It is a more particular object of this invention to provide improved tri-state-type driver circuitry for programmable logic devices.